If it is server based, it really does not need a party system. On a server based online system, you would just look for available servers, join one, send an invite to your friends, hope the server does not fill up, and then game on. Then, once the match is over, depending on how they implemented it, the teams would be randomized, and you would hope your friends are on your team.
The party system is only really necessary if there is a matchmaking system. Matchmaking takes you and your party and matches you against other different parties after every match, and base the match ups on a ranking system. This is what Halo 2 implemented and M$ thought it was so great that they embedded it directly into Live's backbone. This is much easier to implement with a p2p system.
It is mainly a preference over these two systems as to whether parties are necessary. I have not played KZ2 so I do not know if it is server based or matchmaking based.







